Conventional holographic interferometry is known for measuring the surface displacement of articles under test. That technology requires a very precise set-up of optical equipment. A laser is used to illuminate an object and to illuminate a photographic film which is further illuminated by light reflected from the object, and interference of the direct and reflected light at the film produces an interference pattern containing much information about the surface position. By double exposing the film to such light before and after some deformation or movement of the surface, and developing the film, a hologram is produced. Upon reconstruction interferences fringes reveal the amount of surface movement between exposures.
Due to a number of drawbacks, another measurement: technique became desirable, especially in production facilities. The conventional holography is very sensitive to vibration or other movement of the whole object so that special care is necessary. Also the equipment requires very accurate alignment. In addition, a high degree of coherence of the laser light is needed. Another type of interferometry has become more practical. Shearography or shearing interferometry is now widely used, particularly for such tasks as inspecting automotive and aircraft tires.
Shearography entails exposing an object to laser light and making two simultaneous exposures of a film by passing the reflected light through two different optical paths to the film, each path having a focusing lens so that two slightly displaced images of the object are obtained to establish a speckle interference pattern. By stressing the object and repeating the exposure process, a double or multiple exposure is made which, when developed, reveals fringe patterns in areas of local surface strain. The method is relatively insensitive to rigid body displacement and requires a relatively low coherence laser beam. The U.S. Pat. No. 4,139,302 to Hung et al, entitled "Method and Apparatus for Interferometric Deformation Analysis", describes the shearography method, the apparatus used to make a shearogram transparency, and a viewer apparatus for viewing the transparency to observe the fringe pattern.
After a shearogram is made, it is desirable to view it in an optimal manner for analysis of the test results. To this end, white light is projected through a transform lens and the shearogram transparency to image the fringes on a focal plane. Typically, a video camera views this image and displays it on a monitor. The placement of the white light source is determined for theoretically optimum fringe production in accordance with the ideal geometry of the shearogram generating apparatus. However, a number of practical matters force departure from ideal geometry and from ideal photographic conditions, and as a result the viewing device may be unable to deliver the fringed images as clearly and unambiguously as desired.
In the case of tire inspection, for example, inspection for ply separation and other inner defects is made by observing, via shearographic optics, deformation of the inside surface of the tire carcass as ambient pressure is changed. In a production facility the rate of testing and variations in individual tire characteristics does not leave unlimited time to set up each tire for consistent photography in the test station. Moreover, the sizes and physical characteristics of tires in successive tests may vary dramatically, and it is not desired to setup the system at each change. If the tire is not centered in the test station, there will be different object distances involved from image to image and even within each image. Other differences arise such as the color of the tire or its viscoelastic stretch or as the optics are tilted up or down to view different portions of the tire carcass. A fixed viewer geometry has no way of compensating for departures from ideal conditions in the making of the shearogram. Such compensation is desirable whenever it can result in a clearer, higher quality and more detailed image of the pattern.
Because a shearogram comprises a set of fringes superimposed on a photographic image of the object, another source of imaging problems is the color of the tire surface or variations of color due to the effects of mold release material on the surface color, for example, or the presence of debris inside the tire. Suspicious artifacts in the shearogram image may appear to be the result of a tire defect when, in fact, they are caused by coloration or harmless foreign material. It is thus desirable to be able to inspect the tire surface in the absence of the fringe patterns to discern whether the tire surface appearance gave rise to a fringe-like artifact or, alternatively, to remove the photographic portion of the image and leave only the fringe pattern.